Polyphenylene ethers have been used widely as a material for products or parts in the fields of electrical or electronic fields, automotive fields, other industrial material fields, and food or packaging fields because they are excellent in workability and productivity, and can be produced as products or parts of a desired shape by melt injection or melt extrusion molding method.
A number of preparation processes of polyphenylene ethers have been proposed, for example, by Japanese Patent Publication No. Sho 36-18692 and U.S. Pat. Nos. 3,306,875, 3,344,116, and 3,432,466.
In these known publications, the monomer regarded as the most industrially important one is 2,6-dimethylphenol. By using this monomer, polyphenylene ethers having a very important role from the industrial viewpoint can be prepared.
In these many publications, 2,6-dimethylphenol has been used as a raw material without particular attention to impurities therein.
A technique paying attention to the impurities in 2,6-dimethylphenol is disclosed in an Example of Japanese Patent Laid-Open No. Hei 11-286542. Described specifically, a reduction in the content, in 2,6-dimethylphenol to be used for the polymerization, of alkylphenols other than 2,6-dimethylphenol to less than 0.3 wt. % has an effect of improving the activity. However, the above-described disclosure does not reveal whether or not high-molecular-weight polyphenylene ethers which are essentially necessary can be prepared, but only shows that an improvement in an initial oxygen absorption rate is observed, which is apparent to one of ordinary skill in the art. In addition, Japanese Patent Laid-Open No. Hei 11-286542 does not seem to include definite measures to improve the quality (particularly, color tone) of polyphenylene ethers.
Japanese Patent Laid-Open No. Sho 57-177018 describes a preparation process of polyphenylene ethers which comprises polymerizing, by continuous polymerization, 2,6-dimethylphenol containing, as impurities, 0.1 wt. % or less of m-cresol, 0.25 wt. % or less of o-cresol (component a), and 0.15 wt. % or less of p-cresol and/or 2,4,6-trimethylphenol (component b), the ratio (a/b) of components a and b falling within a range of from 0.5 to 25. As is apparent from the description, examples and comparative examples in this publication, all the impurities have a great influence on the molecular weight distribution of the resulting polyphenylene ethers and the precipitation ratio of dichloromethane. Hence, it is not clear from the disclosure of this publication which impurity will materially affect the above-mentioned molecular weight distribution or precipitation ratio. Furthermore, there is completely no information therein concerning the color tone possessed by the polyphenylene ethers.
It can be easily assumed from the conventional processes that, depending on the preparation process of 2,6-dimethylphenol to be used for polymerization, the amount of impurities differs and therefore, the polymerization activity or the molecular weight distribution of polyphenylene ethers cannot be controlled as desired. Consequently, it cannot be considered that the quality (particularly, color tone) of polyphenylene ethers has been fully examined.